Silicoaluminophosphates (SAPOs) and aluminophosphates (ALPOs), are important classes of molecular sieve and have been used as adsorbents and catalysts. As catalysts, SAPOs and ALPOs have been used in processes such as fluid catalytic cracking, hydrocracking, isomerization, oligomerization, the conversion of alcohols or ethers, and the alkylation of aromatics. In particular, the use of SAPOs and ALPOs in converting alcohols or ethers to olefin products, particularly ethylene and propylene, is becoming of greater interest for large scale, commercial production facilities. In this regard silicoaluminophosphates are favoured catalysts for the conversion of oxygenates to olefins with SAPO-34 being of particular importance.
It has been recognised for some time that SAPO and ALPO molecular sieves are relatively difficult to manufacture. SAPO and ALPO synthesis procedures are particularly sensitive to small variations in the composition of the reaction mixture and the conditions used. The problems associated with the synthesis of these molecular sieves are exacerbated by the generally low yields of crystalline material obtained from the synthesis process. These difficulties contribute to the relatively high cost of SAPO and ALPO molecular sieves.
There have been varying attempts in the art to improve the yield of SAPO and ALPO molecular sieves. U.S. Pat. No. 5,296,208 describes a process for the synthesis of crystalline molecular sieve materials including SAPO and ALPO molecular sieves. The process described therein requires the removal of at least some of the nitrogen-containing organic templating agent from the reaction mixture during the crystallization process. This removal has the effect of increasing the final yield of crystalline molecular sieve.
PCT Publication No. WO 01/25151 describes a process for the synthesis of SAPO molecular sieves in which the pH of the synthesis mixture is adjusted during the synthesis. The control of pH in this process results in an improved yield of SAPO molecular sieve.
A further difficulty encountered with SAPO and ALPO molecular sieves is their relative instability in the synthesis mixture once the crystallization reaction is completed. If these materials remain in the synthesis solution for any period of time after the reaction is completed they are prone to dissolution back into the reaction mixture. This effect further reduces the yield of these molecular sieves. This is an important problem in the preparation of SAPO and ALPO molecular sieves as it is often necessary to retain the crystallized molecular sieve within the reaction mixture as a slurry after completion of the crystallization for extended periods of time prior to transfer of the slurry to the recovery stage of the synthesis.
The problem of SAPO and ALPO dissolution is recognised in the art and various solutions have been proposed to reduce this problem in U.S. Pat. No. 5,296,208. One solution proposed is to remove organic templating agent prior to cooling of the reaction mixture. A further proposed solution is the rapid dilution of the reaction mixture with relatively large amounts of water on completion of the reaction. Another proposed solution is the rapid and significant decrease in temperature on completion of the reaction.
There is a need for alternative synthetic processes, which address the problem of SAPO and ALPO dissolution upon storage prior to recovery after completion of crystallization.